MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Fan Relay

Is there an advantage to adding a relay to the cooling fans on a late model MGB?
Jimmy Chew

Yes Jimmy, it saves burning out the contacts in the Otter switch, and gives lower resistance, so higher current, so more cooling. This makes sense as the engine and radiator will have grown silt and corrosion and will probably need a bit more help to keep to the right temperature than when they were new.

David. 78 BGT with relays for lights and fan.


D Balkwill

That's on a V8, which has a relay as standard. The 4-cylinder has a different switch which doesn't seem to mind driving the twin fans the US got on the 4-cylinder, let alone the single fan in the UK. However current (pun not intended) switches for brake lights are so poor that they need a relay, and the relay needs a diode to quench the back emf or even that welds the contacts. The biggest problem with the V8 is that the fans are powered off the green circuit even though it has a relay, and that is powered from the ignition switch and is already heavily loaded. On the 4-cylinder it comes direct off the ignition relay i.e. brown circuit with its own fuse. Using a different relay and powering the fans from the brown makes a big difference on the V8, less so on the 4-cylinder. You will get more benefit from providing local grounds at each relay mounting bracket where it attaches to the body, as there is only a puny black wire going all the way back to the fusebox area, which is used for the headlights as well!

And if you radiator is silted up, then clean it out! Anything else is DPO territory!
Paul Hunt

I wired my fans (the standard thermal switched one, and the extra manually switched one) through relays direct from the starter solenoid. They definitely run faster.

I also moved the supply for the thermal switch from the ignition live to the permanent live.

This makes the main fan start up about a minute after I switch off, and run for about 10 minutes. This makes it much easier to restart after a short stop, although I suppose that there will be some undesirable secondary effect, like the block will crack from having cold water in the radiator.
Martin Layton

Personally I wouldn't have mine able to run with the ignition off, in case something went wrong while parked. I know some modern cars do it, but then some French cars have been known to burn down houses. You shouldn't be having a hot-start problem either, really. Any efefcts from the small amount of gravity circulation caused by a fan cooling the radiator while the engine is stopped is significantly less than when the pump is forcing the circulation, and a lot less than the radiator-full of cold water suddenly entering the engine when the stat first opens.
Paul Hunt

This thread was discussed between 11/04/2009 and 12/04/2009

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.